Whitney, Whitney, a thousand times Whitney.
Jasmine is just.. not a threat, since by that stage you'll probably have your starter in it's final form, and a few other things that could take down Steelix. The Magnemite aren't a real threat anyway, IMO.
Whitney on the other hand, was a nightmare when I was a kid. She was at a point in the game JUST when you were getting quite comfortable with your team, with a good ol' starter to kick anything's ass. This is how I felt when I first challenged her, then Quilava's moves didn't do all that much.
Then of course, Rollout happened. It's just.. too good. Milk Drink and Rollout made it a force to be reckoned with, whereas Whitney's Steelix was usually a OHKO job.